The Burmese army has captured a key rebel outpost in the conflict in northern Kachin state.

A spokesman for the Kachin Independence Army rebel group says the post fell to the military after coming under heavy artillery fire earlier on Saturday.

"That was the reason it collapsed. Finally we have to abandon that area, that mountain," James Lum Dau, the Thailand-based spokesman for the KIA's political wing, told AFP.

He said it was not clear whether the fighting would now move further towards the town of Laiza, where thousands of civilians are thought to be taking shelter, but vowed that if "Laiza falls, [it] does not mean KIA falls, absolutely".

Some civilians had already started to move, he added, but was unable to give further details.

Rebuke to US

This comes as Burma's Government issued a rebuke to the US over its concerns about the fighting.

Last Thursday, the US embassy issued a statement, saying it "strongly opposes the ongoing fighting" and urged both sides to work towards peace.

But the Burmese Foreign Ministry says the statement implies the army was the sole aggressor and that the comments could cause misunderstanding in the international community.

In a response printed in the state-run English language newspaper New Light of Myanmar, it says the statement suggested "only the Myanmar government and the Tatmadaw (army) launched the offensive" and "did not mention anything about terrorist actions and atrocities committed" by the Kachin Independence Army (KIA).

Tens of thousands of people have been displaced in Kachin state since June 2011, when a 17-year-long ceasefire between the government and the KIA broke down.

The conflict has resulted in civilian casualties, although the exact number killed is unclear.